Electric power steering apparatus

Abstract

An electric power steering apparatus transmits an auxiliary torque from an electric motor to a steering system via a mechanical clutch composed of a plurality of sets of friction clutch mechanisms. At least one of the plural clutch mechanism sets is disengageable in advance to the remaining clutch mechanism set. This means that only a part of the clutch mechanisms can be brought to the disengaged state at an advanced timing. When the selected part of the clutch mechanisms is disengaged, the remaining clutch mechanisms are subjected to an unbalanced force tending to disengage the remaining clutch mechanisms. By the effect of the unbalance force, frictional forces acting in the remaining clutch mechanisms are considerably reduced, so that the remaining clutch mechanisms can be disengaged by a small clutch-disengaging force. Thus, the mechanical clutch as a whole can be disengaged by a smaller manual effort or force than as required when all the clutch mechanisms are disengaged at one time.

What is claimed is: 
     
       1. An electric power steering apparatus including an electric motor for generating an auxiliary torque according to a steering torque, and a plurality of sets of friction clutch mechanisms for transmitting the auxiliary torque to a steering system, said plurality of sets of friction clutch mechanisms comprising: an input member;   an output member;   a plurality of tapering spaces defined between said input member and said output member;   a plurality of locking members movably disposed in said tapering spaces, respectively, for selectively engaging and disengaging said input and output members;   a plurality of position control members for positioning said locking members; and   a plurality of urging members urging said locking members against the adjacent position control members,   wherein the clutch mechanisms of at least one of said plurality of sets of clutch mechanisms are brought to a disengaged state in advance to the clutch mechanisms of the remaining set of clutch mechanisms.   
     
     
       2. An electric power steering apparatus according to claim 1 wherein said position control members are disposed on the same circle and circumferentially spaced from one another, one of said position control members associated with said at least one set of clutch mechanisms has a longer circumferential length than the remaining position control member. 
     
     
       3. An electric power steering apparatus according to claim 1, wherein said at least one set of clutch mechanisms is disengageable by a smaller clutch-disengaging force than that required to disengage the remaining set of clutch mechanisms. 
     
     
       4. An electric power steering apparatus according to claim 1, wherein the tapering spaces associated with said at least one set of clutch mechanisms have a greater taper angle than, the tapering spaces of the remaining set of clutch mechanisms. 
     
     
       5. An electric power steering apparatus according to claim 1, wherein the locking members of said at least one set of clutch mechanisms have a smaller diameter than the locking members of the remaining set of clutch mechanisms. 
     
     
       6. An electric power steering apparatus according to claim 1, wherein the locking members of said at least one set of clutch mechanisms have respective contact surfaces having a smaller frictional resistance than contact surfaces of the respective locking members of the remaining set of clutch mechanisms. 
     
     
       7. An electric power steering apparatus according to claim 1 wherein said at least one set of clutch mechanisms has a pair of contact surfaces engageable with the mating locking members and having a smaller frictional resistance than contact surfaces engageable with the respective locking members of the remaining set of clutch mechanisms. 
     
     
       8. An electric power steering apparatus according to claim 1, wherein the locking members of said at least one set of clutch mechanisms have a smaller diameter than the locking member of the remaining set of clutch mechanisms, and wherein said tapering spaces and said position control members each have the same shape. 
     
     
       9. An electric power steering apparatus according to claim 1, wherein the tapering spaces of said at least one set of clutch mechanisms have a greater taper angle than the tapering spaces of the remaining set of clutch mechanisms, and wherein said locking members and said position control members each have the same shape. 
     
     
       10. An electric power steering apparatus according to claim 1, wherein at least one of said position control members have a different shape than the remaining position control member, and wherein said tapering spaces have the same shape and disposed at regular intervals, and said locking members have the same shape and disposed at regular intervals.